Fuel cells

ABSTRACT

A fuel cell sensor (10) comprises a main body (11) in which are mounted working electrodes (12), counter electrodes (13) and respective contacts (14 to 17). The working electrodes (12) are mounted facing each other to define a sample space between them. The electrodes (12) are electrically interconnected in parallel as are the two counter electrodes (13). This arrangement makes it possible to provide a very large working electrode surface area for a small volume sample space (22).

FIELD OF THE INVENTION

This invention relates to fuel cells and in particular, but notexclusively, such cells which act as sensors for oxidisable componentsin gases.

BACKGROUND OF THE INVENTION

Fuel cells were first invented by Sir William Grove in 1839 and inrecent years have been used in many arrangements for the detecting ofoxidisable components of gases or vapours, for example in breath testingequipment. Essentially the fuel cell comprises a working electrode oranode and a counter electrode or cathode which are separated by anelectrolyte, usually by a porous disc impregnated with an acidicelectrolyte. The electrochemical oxidation of the fuel component in thegas results in the development of an electrical potential differenceresulting in a flow of electrons from the anode to the cathode and thiscurrent and/or potential difference can be detected. One such fuel cellis made by Lion Laboratories Plc.

Although these fuel cells have been successful in a limited field,considerable problems have been experienced both in the time taken forthe fuel cell to consume the oxidisable component in the sample and inthe time taken for the cell to clear so that it is ready to sense afurther sample.

SUMMARY OF THE INVENTION

From one aspect of the invention there is provided a sensor fordetecting oxidisable fuel components in a gas or vapour including a pairof working electrodes facing other to define a sample receiving spacebetween them.

This arrangement of facing working electrodes substantially increasesthe surface area of the working electrode for a given cross-sectionaldimension and hence significantly reduces the time taken for a fuelcomponent to contact and hence react with the working electrode. Itfurther enables the working electrodes to be placed very close to oneanother hence reducing the length of the mean free path available to anyfuel component molecule injected into the sample space before it strikesa working electrode. In traditional designs there can be a significantdead space above the single working electrode.

In a preferred embodiment the working electrodes are electricallyconnected. Further it is preferred that there is a counter electrode foreach working electrode and they will be separated from each other by asuitable electrolytically impregnated body. The counter electrodesshould also be electrically connected, when this is true of the workingelectrodes. The respective sets of electrodes can be connected inparallel or in series. In the former arrangement the electrodes could beviewed as being a single cell in a bent configuration, and such anarrangement is included in the invention and indeed other wrapped aroundarrangemennts may be possible although these may introduceconstructional complexities which limit the closest approach of the twoworking electrode sections.

Preferably the spacing between the working electrodes is between 0.5 mmand 5 mm and a spacing of 1 mm to 2 mm has been found to be a goodcompromise between cell efficiency and constructional simplicity.

The rate of clearing of the cell may be a function of the net load andit has been found convenient to have a net load of approximately 10ohms. The sensor may thus have a load resistor across its output whichis approximately equal to its impedance.

From another aspect the invention consists in a fuel cell having aclosed loop electrical contact for at least one of its electrodes, theloop being substantially circumjacent the operative surface of theelectrode.

Traditionally single wire contacts have been used and these can causelocal resistance problems and manufacturing difficulties. The use of aclosed loop, and preferably annular, contact ensures that there iselectrical contact between the electrode and the contact at at leastsome parts of the contact and removes many localised affects.

The invention also consists in a sensor as described above with theclosed loop contact set out above.

Although the invention has been defined above it is to be understoodthat it includes any inventive combination of the features set out aboveor in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be performed in various ways and a specific embodimentwill now be described by way of example, with reference to theaccompanying drawings, in which:

FIG. 1 is an exploded view of one version of a sensor according to theinvention;

FIG. 2 is a diagrammatic vertical cross-sectional view of the electrodesof the fuel cell illustrating their electrical connections; and

FIG. 3 is the same as FIG. 2, but shows an alternative arrangement ofelectrical connections.

DETAILED DESCRIPTION OF THE INVENTION

A fuel cell sensor 10 comprises a main body 11 in which are mountedworking electrodes 12, counter electrodes 13 and respective contacts 14to 17. The main body is closed off by a top cover 18 and a bottom cover19 and sealed by respective O rings 20 and 21.

The working electrodes 12 are mounted facing each other to define asample space 22 between them and a sample inlet 23 debouches into thisspace and an outlet 24 is also connected into it. Each of the electrodes12,13 comprises a platinum black layer on a micro-porous PVC body andthe two bodies 25 in each working/counter electrode pair togethercontain an acidic electrolyte such as H₂ SO₄.

As can be seen from FIG. 2 the two working electrodes 12 areelectrically interconnected in parallel, as are the two counterelectrodes 13. These connections take place through the respectivecontacts 15 to 17 which are each in the form of gold plated stainlesssteel annulus which extends around the peripheral margin of itsrespective electrode.

As has been explained above this configuration allows the workingelectrodes 12 to be brought very close to each other so that the samplespace 22 has a very small volume but a very large working electrodesurface area. This results in a sensor which is extremely sensitive,fast to react and which can also clear rapidly.

These characteristics not only mean that a highly sensitive fuel cell isavailable for traditional existing uses, but also that it can be usedfor less common purposes such as in gas chromatography (as for exampleshown in FIG. 2). In that case the cell is designed to perform best witha carrier or make up gas flow through the cell; the sample gas beingintroduced into this flow. Preferably the make up gas has a 50%-60%relative humidity to prevent drying out of the cell and conveniently itmay be nitrogen.

As previously stated, the electrode pairs are connected in parallel andthis configuration maximises the current generated.

An alternative series connection is shown in FIG. 3. This arrangementminimises current but maximises the electrical potential of the cell.The configuration chosen would then depend on whether the cell is to beused in either a voltage or current measuring device.

We claim:
 1. A fuel cell for detecting oxidizable fuel components in agas or vapor sample comprising a first and second working electrodesfacing each other to define a sample receiving space between them, and acounter electrode for each working electrode, said first workingelectroded being electrically connected to one of said second workingelectrode and its associcated counter electrode.
 2. A fuel cell asclaimed in claim 1, wherein the working electrodes are electricallyconnected to each other.
 3. A fuel cell as claimed in claim 1, whereinan electrolytically impregnated body separates each working electrodefrom its associated counter electrode.
 4. A fuel cell as claimed inclaim 1, wherein the counter electrodes are electrically connected toeach other.
 5. A fuel cell as claimed in claim 1, wherein the sets ofelectrodes are connected in parallel or in series.
 6. A fuel cell asclaimed in claim 1, wherein the spacing between the working electrodesis between 0.5 mm and 5 mm.
 7. A fuel cell as claimed in claim 6,wherein the spacing is between 1 mm and 2 mm.
 8. A fuel cell as claimedin claim 1, wherein the net load for the cell is approximately equal to10Ω.
 9. A fuel cell as claimed in claim 1, wherein the fuel cellincludes a load resistor across its output which is approximately equalto its impedance.
 10. A fuel cell as claimed in claim 1, having a closedloop electrical contact for at least one of its electrodes, the loopbeing substantially circumjacent the operative surface of the electrode.